1. Field of the Invention
The present invention relates to a resin substrate for an optical recording medium and more specifically to a metal mold for making the same by injection molding.
2. Discussion of Related Art
Conventionally, optical recording mediums such as compact discs wherein the substrate is made of an injection-molded resin have been known.
FIG. 10 is a cross sectional view showing an example of metal molds used for injection molding a resin substrate, providing a stamper 2 reinforced by a backing plate 1, a ring-shaped stamper inner circumference presser 22, a ring-shaped stamper outer circumference presser 4, and a mirror plate 5 detachably engaged with the open end of the stamper outer circumference presser 4, to form a disc-shaped cavity 6, which together with the backing plate which serves as a metal mold for supporting the stamper 2.
The above stamper 2 is a thin plate-shaped metal mold on which a reverse pattern 7 of a spiral or concentric signal pattern to be transferred onto a resin substrate is formed and mounted on the backing plate 1 by the stamper inner circumference presser 22 and the stamper outer circumference presser 4.
The stamper inner circumference presser 22 firmly fixes the inner circumference of the stamper 2 to the backing plate 1, whereas the stamper outer circumference presser 4 is arranged so as to provide a clearance 8 of about 15 .mu.m maximum between a stamper abutting surface 4a of the outer presser 4 and the surface of the stamper 2 so that the stamper 2 can be thermally expanded in radial directions freely and a signal pattern having highly true roundness can be transferred by exhausting a gas from the cavity 6 when it is filled with resin, and thus the stamper outer circumference presser 4 loosely supports the outer circumference of the stamper 2.
The cavity-side surface of the stamper outer circumference presser 4, i.e., the surface 4b which forms the outer circumferential surface of the resin substrate, is substantially vertically formed with respect to the surface of the stamper 2 on which the reverse pattern is formed to make the outside appearance of the formed substrate look good.
Further, as shown in FIG. 11, the metal mold is provided with a robot 9 for removing the substrate so that the injection-molded resin substrate 10 can be automatically removed from the cavity 6. The substrate removing robot 9 is inserted between the stamper outer circumference presser 4, after injection molding has been completed and the mirror plate 5, the surface of which in contact with the resin being molded is polished like a mirror, is removed. The robot 9 attaches itself substantially to the center of the resin substrate 10 and pulls it out of the open end side of the stamper mad (in the direction of arrow A).
The known technique regarding the above is disclosed in Japanese Patent Kokoku (Post Exam. Publication) No. Sho 60-18527.
As described above, the metal mold used for the molding of a resin substrate for an optical recording medium is provided with the clearance 8 between the stamper 2 and the stamper outer circumference presser 4, which is different from a usual metal mold, and thus when resin is injected into the cavity 6, melted resin enters the above clearance 8 by pressure produced during the injection and burrs 11 are formed around the circumference of the resin substrate 10, as shown in FIG. 12. The burrs 11 are not uniformly made around the circumference of the resin substrate 10, but partially made or not made at all depending upon the dimension of the above clearance 8.
Therefore, a problem arises in that the resin substance 10 has an inferior outside appearance and thus the commercial value is lowered as well as various functional problems arise in that the resin substrate 10 is liable to come into collision with a device such as, for example, a disc cartridge or an optical recording medium drive unit which moves relative to the optical recording medium, because an external dimension of the resin substrate 10 is made larger than a designed value.
Further, although the effect of the burrs 11 can be avoided in such a manner that when the injection-molded resin substrate 10 is removed from the cavity 6, the combined member of the backing plate 1 and the stamper 2 is separated from the stamper outer circumference presser 4 and the resin substrate 10 is removed from the stamper 2, a disadvantage arises in this case in that the stamper 2 is deformed by a removing force of the resin substrate 10, because the outer circumference of the stamper 2 is provided as a free end. Consequently, as shown in FIGS. 11 and 13, to remove the resin substrate 10, the mirror plate 5 must be separated from the stamper outer circumference presser 4, and the resin substrate 10 is drawn out upwardly along the stamper outer circumference presser 4.
To enable the resin substrate 10 to be removed from the cavity 6 by this method, the resin substrate 10 must be curved to an angle at which the extreme ends of the burrs 11 pass over the corner defined by the stamper abutting surface 4a of the stamper outer circumference presser 4 and the cavity side surface 4b, as shown in FIG. 13. Moreover, in the conventional metal mold mentioned above, since the cavity side surface 4b of the stamper outer circumference presser 4 is formed substantially vertically with respect to the reverse pattern forming surface of the stamper 2, a bending stress is continuously applied to the resin substrate 10 while the extreme ends of the burrs 11 are held in abutment against the cavity side surface 4b of the stamper outer circumference presser 4. Various obstacles arise when the resin substrate 10 is removed from the stamper outer circumference presser 4 while such a bending stress is applied to the resin substrate 10. That is, a problem arises in that the flat shape of the resin substrate 10 is liable to be distorted and the resin substrate 10 is susceptible when being made into an optical anisotropic member. In particular, since the resin substrate 10 not be removed from the metal mold before it has been sufficiently cooled to increase productivity, such a problem is often the result.
When the resin substrate 10 is removed from the metal mold at a temperature as high as possible but below the thermal deformation temperature thereof (accordingly, at a temperature near to the thermal deformation temperature), the characteristics of the resin substrate with respect to optical anisotropy, such as the retardation (birefringence) of the resin substrate, can be kept in a good state, however the drawback arises in that the resin substrate is deformed when it is removed from the metal mold and it remains deformed, as shown in FIGS. 11 and 13. This residual deformation includes such types as:
(i) a residual curvature as shown in FIGS. 14 and 15, which is often observed in a resin substrate in which the burrs shown in FIG. 13 are uniformly formed around the entire circumference thereof having substantially the same length; and PA1 (ii) an upward and downward waving curvature, which is formed on the surface of the resin substrate, when the above burrs 11 having varying lengths are formed around the entire circumference of the resin substrate.
In particular, as the cycle of the waves is made shorter in the second circumstance, a problem arises in that the sensitivity of the optical disc for recording and reproducing information is greatly lowered and the recording and reproducing error ratio is increased when the optical disc is used. The degree of adverse effects applied to the optical disc product by the residual deformation of the resin substrate depends upon an angle .phi. representing a magnitude of the deformation shown in FIG. 14, and, in general, the sensitivity for recording and reproducing information deteriorates and the information recording and reproducing error ratio is increased in the range of .phi.&gt;5m radian.
Therefore, the temperature at which the injection-molded resin substrate is removed from the metal mold is needed to be a temperature at which the above deformation does not remain. This temperature is different depending upon the kind of a resin material, and the time necessary from supply of resin to the cavity of an injection molding machine to removing the melted resin is also different depending upon the kind of resin, and thus a problem arises in that the manufacturing job is complex and the manufacturing cost is increased.